The use of thermal barrier coatings (TBC) on components such as combustors, high pressure turbine (HPT) blades, vanes and shrouds is increasing in commercial as well as military gas turbine engines. The thermal insulation provided by a TBC enables such components to survive higher operating temperatures, increases component durability, and improves engine reliability. TBC is typically a ceramic material deposited on an environmentally-protective bond coat to form what is termed a TBC system. Bond coat materials widely used in TBC systems include oxidation-resistant overlay coatings such as MCrAlX (where M is iron, cobalt and/or nickel, and X is yttrium or another rare earth element), diffusion coatings such as diffusion aluminides that contain aluminum intermetallics.
Ceramic materials and particularly binary yttria-stabilized zirconia (YSZ) are widely used as TBC materials because of their high temperature capability, low thermal conductivity, and relative ease of deposition such as by air plasma spraying (APS), flame spraying such as hyper-velocity oxy-fuel (HVOF), physical vapor deposition (PVD) and other known TBC application techniques. TBCs formed by these methods generally have a lower thermal conductivity than a dense ceramic of the same composition as a result of the presence of microstructural defects and pores at and between grain boundaries of the TBC microstructure.
TBCs employed in the highest temperature regions of gas turbine engines are often deposited by electron beam physical vapor deposition (EBPVD), which yields a columnar, strain-tolerant grain structure that is able to expand and contract without causing damaging stresses that lead to spallation. Similar columnar microstructures can be produced using other atomic and molecular vapor processes, such as sputtering (e.g., high and low pressure, standard or collimated plume), ion plasma/cathodic arc deposition, and all forms of melting and evaporation deposition processes (e.g., laser melting, etc.).
Under service conditions, these TBC coated hot section engine components can be susceptible to various modes of damage, including erosion, oxidation and corrosion from exposure to the gaseous products of combustion, foreign object damage and attack from environmental contaminants. The source of the environmental contaminants is ambient air, which is drawn in by the engine for cooling and for combustion. The type of environmental contaminants in ambient air will vary from location to location, but can be of a concern to aircraft as their purpose is to move from location to location. Environmental contaminants that can be present in the air include sand, dirt, volcanic ash, sulfur in the form of sulfur dioxide, fly ash, particles of cement, runway dust, and other pollutants that may be expelled into the atmosphere, such as metallic particulates, such as magnesium, calcium, aluminum, silicon, chromium, nickel, iron, barium, titanium, alkali metals and compounds thereof, including oxides, carbonates, phosphates, salts and mixtures thereof. These environmental contaminants are in addition to the corrosive and oxidative contaminants that result from the combustion of fuel. However, all of these contaminants can adhere to the surfaces of the hot section components, which are typically thermal barrier coated.
In order for a TBC to remain effective throughout the planned life cycle of the component it protects, it is important that the TBC has and maintains integrity throughout the life of the component, including when exposed to contaminants. Some contaminants may result in TBC loss over the life of the components. For example, calcium-magnesium-aluminum-silicate (CMAS) particulates are often contained in the atmosphere of areas having fine sand and/or dust. CMAS infiltration is a phenomenon that is linked to thermal barrier coating (TBC) spallation in hot section turbine components. A typical composition for CMAS includes a low melting point deposit having about 35 mol % CaO, about 10 mol % MgO, about 7 mol % Al2O3, about 48 mol % SiO2, about 3 mol % Fe2O3 and about 1.5 mol % NiO. Surfaces operating at temperatures of greater than about 2240° F. (1227° C.) may come into contact with CMAS, which becomes a liquid and infiltrates into the columnar structure of the TBC. The CMAS interferes with the compliance of the columnar structure of the TBC resulting in spallation and degradation of the TBC. In addition, CMAS may infiltrate into dense vertically cracked TBC or into the horizontal splat boundaries of thermally and plasma sprayed microstructures and cause spallation and/or other degradation to the TBC structure. In addition to the compliant loss, deleterious chemical reactions with yttria and zirconia within the TBC, as well as with the thermally-grown oxide at the bond coating/TBC interface, occur and result in a degradation of the coating system. Continued operation of the engine once the passive thermal barrier protection has been lost leads to oxidation of the base metal superalloy protective coating and the ultimate failure of the component by burn through cracking.
An attempt to mitigate the affect of the CMAS on high pressure turbine blades has been to apply a thin layer of aluminum (Al2O3) on the TBC to increase the melting point of CMAS about 100 to 150° F. (38° C. to 66° C.). The addition of the aluminum oxide provides an increase in operating temperature of up to about 2400° F. (1316° C.) with reduced infiltration of liquid CMAS. However, grinding during manufacture and assembly, as well as grinding and rubbing during gas turbine engine operation of a turbine shroud make the addition of an aluminum oxide layer difficult and impractical as well as provides additional manufacturing cost and complexity, wherein turbine blades which are subjected to gas and particle erosion would have different requirements for aluminum oxide overcoating and concern about eroding away of the alumina coating. In addition, thicker alumina layers are subject to coefficient of thermal expansion mismatches within the TBC coating system, resulting in thermal strains during cycling.
What is needed is an improved system and method for providing resistance to contaminants, such as CMAS, to gas turbine engine components that operate at temperatures above the melting temperatures of the contaminants.